A printing apparatus used as the printing section of a printer, copier or facsimile machine or a printing apparatus used as the output unit of a multifunction electronic device, which includes a computer or word processor, or of a work station is adapted to print an image on a printing medium such as printing paper or thin plastic sheet based upon image information. Such printing apparatus can be classified according to their printing methods, e.g., ink-jet printers, wire-dot printers, thermal printers and laser printers, etc.
In a serial-type printing apparatus in which an image is printed on a printing medium by moving a printhead back and forth in a direction (referred to as the "main scan direction") that intersects the direction in which the printing medium is conveyed (this direction is referred to as the "sub-scan direction"), printing is performed over the entire printing medium by repeating an operation which includes printing an image by the printhead mounted on a carriage, conveying the printing medium a predetermined amount whenever the printing of one scan (one line) is completed and then printing the next line of the image on the printing medium whose conveyance has been halted.
FIG. 9 is a diagram illustrating an example of printing position precision produced when a printhead has printed a given line (where a dot is printed every 1/90 of a pitch and the deviation is measured).
In FIG. 9, the horizontal axis indicates the positions of printed dots, in which the position at which printing starts is taken as the starting point. The vertical axis indicates cumulative amount of deviation from a position at which printing was supposed to have been performed. The positive side of the vertical axis indicates deviation in a direction away from the home position of the printhead, where the position at which printing is supposed to be performed is taken as zero, and the negative side indicates deviation in a direction toward the home position.
Since printing usually starts after the carriage motor has attained a constant speed, the carriage motor starts being driven somewhat short of the position at which printing is to start, thereby moving the carriage. Accordingly, as will be understood from FIG. 9, printing precision is somewhat poor immediately after start-up of the carriage motor that drives the carriage (i.e., in the vicinity of the printing starting point), and the deviation in the printing position at this time is .+-.40-50 .mu.m. The printing position subsequently stabilizes and the amount of deviation becomes .+-.10-20 .mu.m.
Accordingly, in a case where staircase-like ruled lines of the kind shown in FIG. 10, for example, are printed on a printing medium P, the printing starting position differs for each scan of the printhead and the position at which the carriage motor is driven changes little by little at the locations of the steps of the ruled lines. As a consequence, the position at which movement of the carriage starts shifts from that of the preceding line of scanning.
This will be described with reference to FIG. 9.
The curved formed by the black dots in FIG. 9 indicates the change in amount of deviation of printing position produced on a given line in a case where staircase-shaped ruled lines are printed. The curve indicating the phantom line represents the change in amount of deviation of printing position produced on the immediately preceding line in printing of the staircase-shaped ruled lines. The reason why the curve indicated by the phantom line starts from an area on the negative side of the horizontal axis is that the printing starting point on the curve formed by the black dots is taken as the starting point and the staircase ruled line of the preceding line is shifted to the left side by an amount equivalent to this step of the staircase.
In any case, when a staircase-shaped ruled line is printed in this fashion, there are instances where a maximum deviation of 70 .mu.m occurs between neighboring printed lines.
Such deviation becomes conspicuous where the ruled lines join, namely at positions a, b, c and d in FIG. 10, and causes a decline in the image quality of the printed image.
In a printing apparatus that forms the color black by superimposed printing of printing materials of multiple colors Y (yellow), M (magenta) and C (cyan), the above-mentioned deviation is a cause of color offset and results in a marked decline in image quality.
As arrangement has been proposed in order to solve this problem. For example, as described in the specification of Japanese Patent Publication Laid-Open No. 9-118057, each scanning starting position of a carriage is adjusted so as to take on a distance that is an integral multiple of distance the carriage is moved by rotation of a stepping motor resulting from one cycle of the excitation phase of the motor. The purpose of this is to eliminate a variance in printing position at the start of printing.
However, except for instances where cogging of the carriage motor is pronounced, vibration of mechanical portions is dominant in terms of amount of deviation from the absolute position of the printing starting point shown in FIG. 9. This makes necessary measures for dealing with vibration. Such vibration exhibits a characteristic period.
Measures for dealing with this problem in the prior art as indicated below.
(1) An encoder is mounted on the printing apparatus and the absolute position of the carriage is detected to assure precision in terms of the positions at which the dots of an image are formed.
(2) Since the carriage often is driven using a stepping motor, initially the motor is started up at low rpm and is accelerated to a prescribed usable rpm. After the motor attains this rotational speed, printing is carried out. If the carriage is subsequently stopped, rotation of the motor is slowed down from the usual rpm and is halted once the low rotational speed is attained. Lengthening distance at start-up of the motor reduces a fluctuation in carriage speed at the rpm used in printing and assures precision at which dots are formed into an image.
With the examples of the prior art mentioned above, however, extra components such as the encoder are required, thereby raising equipment cost. Further, lengthening distance at motor start-up leads to a reduction in effective printing length or, if a satisfactory printing length is maintained, to an increase in the size of the apparatus per se.